WASHINGTON (CBS/AP) — With members of the largest fire fighters chanting “Run Joe, run,” former Vice President Joe Biden on Tuesday teased out themes of a possible presidential candidacy, criticizing President Donald Trump’s leadership while questioning Republicans’ commitment to middle class America

Biden, a Democrat, has not announced a presidential bid, but he is in the final stages of preparing for a 2020 presidential campaign, CBS News reports. He asked members of the International Fire Fighters Association gathered in Washington to save their energy, because “I may need it in a few weeks.”

WATCH: Emphatic chants of "Run, Joe, run!" greeted former VP Joe Biden as he addressed the International Association of Fire Fighters Tuesday; "save [your energy] a little longer," he quipped, hinting at a possible 2020 bid. "I may need it in a few weeks" https://t.co/zlEu1BOujUpic.twitter.com/4bJ4h4sw7t

During a speech that lasted about 30 minutes, Biden repeatedly tore into Republicans — and Trump — as supporting policies that favor the wealthy to the detriment of the middle and working classes.

He questioned those who oppose collective bargaining rights, saying: “Who the hell are these guys?”

And he singled out the Republican-championed tax cut signed into law by Trump, arguing that Republicans want to pay for it with devastating cuts to the social safety net.

“I’ll never understand people who fight you on the basics, like safety, fair play health care,” Biden said.

He expanded his critique of Trump to broader themes about the country’s founding principles.

“We can’t be divided by, race religion, by tribe. We’re defined by those enduring principles in the Constitution, even though we don’t necessarily all know them. In America everybody gets a shot,” Biden said. He added: “That’s what I don’t think this current president understands at all.”

He also questioned Trump’s “America First” foreign policy.

“Who fills the vacuum if we continue to walk way and walk off the world stage?” he asked. “This is America, so it’s time to get up remember who the hell we are.”

Biden has also told friends that he thinks he is the Democrat’s best chance to beat President Donald Trump in 2020.

Biden’s campaign is expected to launch next month.

NEW: Former VP Joe Biden is in the final stages of preparing for a 2020 presidential campaign that is expected to launch next month, @edokeefe reports; "he's much more confident than he was previously," one person familiar with Biden's plans says. pic.twitter.com/pPARO7QcBm